U.S. Pat. No. 5,107,698 which issued to Gilliam in 1992 discloses a smoke generating apparatus. Smoke generating apparatus, such as those disclosed by Gilliam, are used to test for hairline cracks and similar leaks that are not detectable by visual inspection. The location of the leak is detected by observing smoke exiting from the leak. The Gilliam apparatus has a container in which is disposed a heating element. The container is filled with a smoke producing liquid until the liquid level reaches, without submerging, the heating element. An air pump forces a flow of air to bubble up through the smoke producing liquid, propelling the smoke producing liquid into contact with the heating element. Any of the smoke producing liquid coming in contact with the heating element is vaporized. When the smoke producing liquid is vaporized, it becomes smoke. As the flow of air exits the container it carries the smoke. The flow of air can be directed as required for testing purposes.
The Gilliam apparatus has limitations due to incomplete combustion. Particles of partially burned solution and unburned solution become entrained in the air flow and potentially can temporarily plug the leaks that the apparatus is supposed to be detecting.